A quadrature oscillator that uses LC resonators, each consisting of an inductor (L) and a capacitor (C) connected in parallel, to produce four-phase clocks of different phases by LC resonance is known. Quadrature oscillators may be used, for example, in FSK modulators and Clock and Data Recovery (CDR) circuits that are used for the high-speed interconnection for sending and receiving data at high speed by wired or wireless communication. FSK modulators are circuits that vary frequencies of carrier waves in accordance with data. CDR circuits recover a clock and data from a data signal in which the clock is superimposed.
Widening a variable range (range in which the capacitance varies) of a variable capacitor in an LC resonator widens the range in which a quadrature oscillator oscillates. The wider oscillation range of the quadrature oscillator enables the quadrature oscillator to cover a plurality of oscillation frequencies (for example, 25 GHz and 28 GHz, or 32 GHz and 36 GHz) with one quadrature oscillator without using a plurality of quadrature oscillators for respective frequencies.
The greater variable capacitance, however, decreases the Q value (quality factor) that indicates the performance of the LC resonator, for example, increases the noise in the clock. The increase in the variable capacitance due to the greater variable capacitance may cause unignorable parasitic capacitance and thereby the oscillation range may become narrow.
The following is a reference document. [Document 1] Japanese Laid-open Patent Publication No. 2015-91084.